Learn how to choose between Prebid server-side and client-side (aka browser-based) header bidding to increase your advertising revenue as a publisher and regain control over your programmatic stack.
With the rise of header bidding and Prebid in particular, publishers have seen their advertising stack evolve quite significantly over the last few years. More recently, web publishers and their monetisation teams have been considering making the switch from running their Prebid auctions client-side to running these auctions server-side.
While one of the main arguments in favor of server-side is how it appears to be future-proof, it also seems that the topic of server-side is not “mature” enough for many publishers and that there is still a lot of revenue to be claimed through client-side setups.
In this article, we go over the pros and cons of each alternative and explain why the time to make the full switch to server-side might not be just yet.
But first of all, what is Prebid client-side, you may ask?
Prebid client-side (also known as browser-based Prebid), is the original version of Prebid and currently the most widely used version.
In the case of “client-side”, the auction happens in the user’s browser, with the ad exchanges bidding in real time on the publisher’s inventory. The winning bid is then selected and sent back to the publisher’s ad server to generate the impression of the ad on the page visited by the user.
Not only is client-side considered the easiest to implement for publishers and their monetisation teams, but it is also quite a flexible option since it does not require any changes to the publisher’s ad server.
To sum things up, minimal technical implementation and wide flexibility have made this solution the go-to for publishers looking to generate programmatic revenue.
In order to further streamline the bid process and increase revenue by favoring competition between bidders, Prebid server-side has since been created. It has recently gained in popularity by displacing the auction process from the user’s browser to a separate online server, usually owned and operated by the publisher’s team themselves.
The goal is usually the improvement of Core Web Vitals, such as page speed. These are monitored more closely than ever by Google right now, as Core Web Vitals are used to display AMP inventory based on mobile-inventory instead of real AMP pages. However, server-side header bidding may not be the optimal solution at present. While it addresses latency and page speed concerns, especially with multiple bidders, it also introduces some drawbacks.
Although server-side allows for more demand sources to be connected, it also implies increased costs both to run the server and hire the required professionals to create and operate this server-side setup, both of which come at a certain cost for publishers. And even if you have the budget, finding such professionals is increasingly difficult in today’s ad tech market.
While server-side is newer and aims to improve page speed and user experience, its complexity and high operational costs outweigh its client-side counterpart. Publishers need robust servers and maintenance expenses, which may not outweigh the revenue uplift caused by switching to server-side.
If they do choose server-side, publishers will not only need to invest in servers with sufficient processing power, storage, and bandwidth to handle bids, but also plan for additional maintenance and infrastructure expenses (not to mention hiring the right people to operate this server, which does not come cheap).
On the previously mentioned topic of latency, server-side might actually not have that much of an effect, with client-side bidding showing to be more efficient in terms of latency and requests in a majority of cases nowadays (as long as you don’t have a bottomless list of bidders waiting to shoot their bids, that is). Also, cookie synchronization in server-side setups doesn't yield the same results as in client-side setups. However, we'll delve deeper into this later in the article.
Server-side is scalable and can handle more demand sources, making it attractive for large publishers. However, implementing it requires technical knowledge and manpower, making this switch seem too advanced for a majority of online publishers.
While server-side may seem profitable, it is still a complex and advanced topic for most publishers. The scarcity of skilled ad tech professionals proficient in the required coding language adds to the challenges. Considering the hiring and implementation time, the entry cost of server-side may outweigh its revenue uplift compared to client-side bidding.
Last but not least, server-side lacks the transparency that client-side offers: no visibility on winning bids for server-side and lack of synchronisation of third-party cookies. Although Google announced the presence of 3rd party cookies on Chrome until 2024, publishers should still consider them in their monetisation strategy.
Client-side has an advantage here as demand partners can easily access cookies, simplifying synchronisation. In contrast, server-side acts as a middleman, complicating the auction process and increasing latency. This setup complexity and communication delay can lead to revenue losses originating from discrepancies and data inconsistencies.
So, if you have a limited list of bidders, there’s a good chance that client-side might remain the right move for you!
In conclusion, both Prebid server-side and Prebid client-side have their pros and cons. Prebid server-side seems like a good solution and will probably represent the future of header bidding, but it is not entirely mature just yet.
More importantly, publishers should keep in mind that implementing server-side header bidding is not a simple process, both in terms of technical cost and financial cost (which are related to the operation and maintenance of the aforementioned server)
Publishers also need to keep in mind that there is still a lot of revenue to claim by using client-side, which still remains more efficient than server-side in a majority of cases. In many cases, leveraging other actions impacting your website’s performance (notably page loading speed) will prove to be a quicker and simpler process for monetisation teams compared to the potentially tricky implementation of server-side header bidding.
While smaller monetisation teams may find making the switch to be quite tedious, larger teams with a better programmatic maturity may have the means and budget to definitely change to server-side.
The decision between Prebid server-side and Prebid client-side ultimately depends on the publisher's specific needs and goals. If scalability, reducing latency and maintaining a low list of bidders are top priorities, then Prebid server-side may be the better option.
However, if generating higher revenue through demand competition and flexibility are more important, then Prebid client-side may be the way to go. Regardless of which solution publishers choose, the important thing is to continue using header bidding to maximise your monetisation potential and take advantage of the benefits of programmatic advertising.
If you’ve been reading this article to improve your revenue as a publisher, chances are there may be more than one root behind your website underperforming revenue-wise. One of these potential root causes is for your SSPs to not be connected the way they should, resulting in lower volume of bids (bid density), SSPs not bidding on certain contexts or worse yet, currency issues that may cause differences between the income reported by the publisher, and the income reported by the SSP. We’ve listed a couple of these potential solutions to explore in the following article: How to make sure your SSPs are performing at an optimal level?
However if you are just starting to set up your monetisation strategy and would like to start with the basics, we’ve compiled a checklist gathering all the decisive factors to help you create top-performing ad units. All elements on this list are updated as of 2023 and will help you ensure that your inventory’s auctions have bidders lining up: “How to set up a top-performing Ad Slot in 2023?”